To quote: I've hooked up Showtime to the video decoder libraries already present on the PS3. This will accelerate h264 (Up to level 4.2) and MPEG2 video decoding so it can play up to 1080p content.

If you want to make sure that the Cell acceleration is actually used you can press L3 during video playback to see the status window. Make sure that the video codec mentions (Cell) at the end of the row. Just like in this screenshot.